1. Field of the Invention
The present invention relates to a bipolar transistor, particularly to a heterojunction bipolar transistor.
2. Description of the Related Art
Recently, research and development of heterojunction bipolar transistors (HJBTs), which use a compound semiconductor, has been remarkable. However, it is still difficult to manufacture an integrated circuit having heterojunction bipolar transistors.
One problem is that characteristics of the HJBTs vary from transistor to transistor. For example, in an ECL (emitter coupled logic) integrated circuit, it is important that the pair of driver transistors that constitute a differential amplifier have uniform characteristics. If the characteristics of the transistors are non-uniform, a large current source is necessary to keep a wide operational margin. Accordingly, the deviation of the device characteristics leads to problems in implementation and fabrication of this type of integrated circuit.
A second problem is that it is difficult to further miniaturize HJBTs. Further miniaturization is important to realize high speed devices.
For example, the conventional HJBT may have a very small scale structure in the vertical direction, e.g., less than 10 nm. However, the length of the transistor in a horizontal direction is at least a few hundred nm due to limitations in lithography.
As explained above, the conventional heterojunction bipolar transistor has problems that make it difficult to manufacture a pair of transistors having uniform characteristics. In addition, it is difficult to form very small scale transistors, particularly in the horizontal direction.